The present invention relates to a method and apparatus for detecting and screening foreign matters that might be present in medical fluids, toiletry fluids, chemical fluids, drinks, etc. filled in transparent or translucent ampoules, vials, or bottles.
The present invention will be illustrated by an example of an inspection operation to determine the presence or absence of small particles in ampoules containing a medical fluid.
For a detecting and screening machine that detects small particulate foreign matters that might be present in a medical fluid contained in ampoules and screens the ampoules automatically in place of visual inspection, it is required that only those ampoules which contain particles larger than the standard (hereinafter called large foreign matters) should be screened and rejected and other ampoules which contain particles smaller than the standard (hereinafter called small foreign matters) should be accepted.
Usually, defective ampoules containing large foreign matters account for only a small portion of the total number of ampoules, while acceptable ampoules containing small foreign matters that will not affect the quality account for a large portion in the entire ampoules. This in enhanced more as small foreign matters decrease in size. Consequently, if the detecting sensitivity is increased in an attempt to catch large foreign matters completely, small foreign matters are also caught. This increases the rejected fraction and decreases the productivity.
If, conversely, the detecting sensitivity is lowered in an attempt to decrease the rejected fraction, large foreign matters tend to be overlooked and the quality of the entire lot will be severely degraded.
In an actual example in which the detecting sensitivity was set at a high level, the detection was repeated twice, and any ampoules that reacted to either detection were rejected, nearly all (99.7%) defective ampoules were rejected; at the same time, however, many acceptable ampoules were also rejected and the rejected fraction increased more than 4%. In another example in which the detecting sensitivity was set at a low level, the detection was repeated twice, and any ampoules that reacted to either detection were rejected, the rejected fraction was less than 1% but the ratio of defective ampoules rejected was about 90%.
The method in which the samples are detected once each with the sensitivity set at a high level and at a low level provided an intermediate ratio, which as not yet satisfactory.
In order to satisfy these two contradictory conditions, it is necessary to improve the detecting and screening characteristics (the relationship between the particle size and the reject ratio) so that foreign matters larger than a certain standard can be caught completely.
The detection of small particles in an ampoule is accomplished by turning the ampoule and then bringing it quickly to a standstill, causing the small particles in the fluid to be suspended and swirled. Thus the same particle in an ampoule differs in its position, movement, and posture with respect to the detector, each time when detection is accomplished, and accordingly the output signals to be generated in the detector corresponding to the same particle also varies to some extent. This suggests that large foreign particles might be overlooked if only one detection is provided. For positive detection of small particles, it is indispensable to repeat more than twice the detection of the same ampoule.
On the other hand, small foreign matters will generate signals exceeding the screening standard when the detection is repeated over and over again. For instance, assume an ampoule containing a small particle which will be judged to be unacceptable at a rate of once in one hundred detections. Incidentally, the detection rate of this small particle is 1%. In a given lot there are a large number of ampoules that contain small particles of about 1% detection rate. Therefore, the more the number of detections is increased, the higher will be the defective fraction.